The invention generally relates to apparatus and methods for unloading powder from a container, such as shipping containers or boxes. More specifically, the invention relates to improvements to a powder pick-up tube associated with such apparatus.
As generally discussed in the above incorporated patents, electrostatic powder coating technology can involve the delivery of powder from a container to a spray gun. The container may be, for example, a shipping box or barrel or may be a separate fluidizing hopper. Generally, fluidizing hoppers may incorporate some type of pressurized air fluidization structure, usually including a porous plate or conduit structure disposed at the bottom of the hopper. The fluidizing action of pressurized air directed through the porous plate or conduit structure allows the powder to be drawn through a powder pump and directed to an attached powder spray coating gun.
Alternatively, as disclosed in the ""344 and ""450 patents, vibrating units may be used to vibrate a container or box of powder. This fluidizes the powder enough that the powder may be drawn up through a powder pick-up tube by a powder pump and then directed to the powder spray coating gun. In systems that employ vibrating units, the original shipping container, such as a box lined with a plastic bag, may be placed directly on the vibrating unit. In this instance, there is no need to transfer the powder from the original shipping container to a separate hopper. This is generally desirable in lower production facilities, such as small job shops, which do not require large powder supplies and which have not typically required the higher quality coatings that may be produced with air fluidized systems.
One problem associated with air fluidized systems is that the pressurized air can cause the powder to drift or become airborne above the container or hopper. Despite this drawback, increasing numbers of powder coating equipment users are demanding the consistent coatings or film builds that can be produced by these systems. This desire is being seen even in smaller job shops. To achieve this coating consistency, the powder spray coating gun must receive a consistent delivery of powder and pressurized air fluidization is known to help achieve this consistency.
One of the main problems associated with fluidizing powder in an original shipping container using pressurized air is that a separate pressurized air fluidizing unit or conduit system has been required at the bottom of the container adjacent to the lower inlet end of the pick-up tube. As mentioned above, these pressurized air fluidizing units can lead to powder drift out of the container and, for example, cause associated powder clean up problems. While the user may close off the container at the top, such as by closing the plastic liner or bag around the powder pick-up tube, this is not seen as an adequate solution.
The ""344 and ""450 patents disclose a double-walled powder pick-up tube having a lower closed end to alleviate clogging problems, for example, associated with the plastic liner. Also, a venting aperture and passage, connected to atmosphere, lead to the powder inlet end of the tube also to alleviate powder clogging problems typically experienced on start-up. Another related apparatus is disclosed in U.S. Pat. No. 4,505,623, also assigned to the assignee of the present invention. This apparatus also uses a double-walled powder pick-up tube and discloses the principle of drawing atmospheric air through the space between the two tubes for fluidizing powder at the powder inlet end. Like other prior devices, this apparatus does not adequately address the specific concerns of this invention.
Finally, powder pick-up tube structure including positively pressurized air for fluidization purposes is known. However, such powder pick-up tubes have included a porous annular fluidizing element connected to the exterior of the tube at the powder inlet end. The fluidizing air is therefore directed radially outward around the end of the pick-up tube and this can create various problems including those associated with powder drift from the container as mentioned above.
It would therefore be desirable to provide a powder pick-up tube and a pressurized air fluidization system that alleviates these and other concerns while providing much more consistent powder flow and film build on products.
The present invention therefore provides a powder pick-up tube connected to a source of negative air pressure, such as a conventional powder pump, for suctioning powder from a container. In accordance with the main advantage of this invention, the pick-up tube is further connected to a source of positive air pressure for providing positive air fluidization of the powder from a substantially enclosed and localized fluidizing element as the powder is suctioned into the powder pick-up tube. The source of positive air pressure is preferably the same source of air pressure used for operating a powder spray coating gun associated with the apparatus. In this way, the positively pressurized air may be sent to the powder pick-up tube only during activation or triggering of the spray gun. An air line connects to a passage in the powder pick-up tube and communicates with a location disposed adjacent to a powder inlet at one end of the tube. The tube includes a lengthwise passage therein leading to a powder outlet which is connected to the source of negative air pressure, such as a powder pump.
Preferably, the powder pick-up tube is a double walled tubular structure in which a first lengthwise internal passage is surrounded by a second, annular lengthwise passage. The first lengthwise passage is used as the powder pick-up passage, while the second lengthwise passage is supplied with positively pressurized fluidizing air. This second lengthwise passage has an air inlet and an air outlet. Preferably, the air outlet at least substantially surrounds the powder inlet of the first lengthwise passage.
Also in accordance with the invention, the air outlet preferably includes an air diffusing member, which may specifically take the form of a perforated or porous insert, such as a porous polymer or metal insert. The air diffusing member preferably has an outlet that directs positively pressurized air toward the powder inlet and is constructed and oriented such that the pressurized air is not directed radially outward with respect to the powder inlet in a substantial manner. More specifically, an air outlet surface is disposed generally at an angle directed toward the powder inlet.
As the positively pressurized air is introduced in the above described manner, localized powder fluidization occurs during suctioning of the powder into the tube. Also, movement of powder into the powder inlet of the pick-up tube is assisted by the specifically directed pressurized air. In the preferred embodiment, the pressurized air is almost completely drawn with the powder up the tube and therefore cannot cause powder drift out of the container. A flow regulating device may be connected to the air line that supplies the positively pressurized air proximate the powder inlet. This can ensure that pressurized air is directed to the powder pick-up tube at a preferred pressure range of about 2 psi to about 15 psi. More preferably, the air pressure is supplied to the pick-up tube at about 8-10 psi and the air has a flow rate of between about 0.5 cfm and about 3.0 cfm.
The invention further contemplates a method of supplying powder to a powder spray coater by way of the apparatus and powder pick-up tube structure generally described above. This method generally involves the steps of placing the inlet end of the tube into a container of powder and negatively pressurizing the first lengthwise passageway to draw powder from the container into the powder inlet. In accordance with the invention, positively pressurized air is directed through the air outlet and toward the powder inlet to fluidize the powder as the powder is drawn into the powder inlet. The method further contemplates additional steps including supplying the pressurized air through a diffusing member, such as a perforated or porous material.
These and other objects, advantages of the invention, and specific details of one preferred embodiment will become more readily apparent to those of ordinary skill upon review of the following detailed description taken in conjunction with the drawings.